T1 Digital WAN carrier facility. T1 transmits DS-1-formatted data at 1.544 Mbps through the telephone-switching network, using AMI or B8ZS coding. Compare with E1. See also AMI, B8ZS, and DS-1. T3 Digital WAN carrier facility. T3 transmits DS-3-formatted data at 44.736 Mbps through the telephone switching network. Compare with E3. See also DS-3. TAC 1.) Terminal Access Controller. Internet host that accepts terminal connections from dial-up lines. 2.) Technical Assistance Center. Cisco TACs provide technical assistance to partners and end users, and form the hub of Cisco global support. TACACS Terminal Access Controller Access Control System. Authentication protocol, developed by the DDN community, that provides remote access authentication and related services, such as event logging. User passwords are administered in a central database rather than in individual routers, providing an easily scalable network security solution. See also TACACS+. TACACS+ Proprietary Cisco enhancement to TACACS. Provides additional support for authentication, authorization, and accounting. See also TACACS. tagged traffic ATM cells that have their CLP bit set to 1. If the network is congested, tagged traffic can be dropped to ensure delivery of higher-priority traffic. Sometimes called DE (discard eligible) traffic. See also CLP. TAXI 4B/5B Transparent Asynchronous Transmitter/Receiver Interface 4-byte/5-byte. Encoding scheme used for FDDI LANs as well as for ATM. Supports speeds of up to 100 Mbps over multimode fiber. TAXI is the chipset that generates 4B/5B encoding on multimode fiber. See also 4B/5B local fiber. T-carrier TDM transmission method usually referring to a line or cable carrying a DS-1 signal. TCP Transmission Control Protocol. Connection-oriented transport layer protocol that provides reliable full-duplex data transmission. TCP is part of the TCP/IP protocol stack. See also TCP/IP. TCP/IP Transmission Control Protocol/Internet Protocol. Common name for the suite of protocols developed by the U.S. DoD in the 1970s to support the construction of worldwide internetworks. TCP and IP are the two best-known protocols in the suite. See also IP and TCP. TCS Test and control system. Independently-powered subsystem used to initialize, monitor, and troubleshoot the hardware on a LightStream 2020 ATM switch. The TCS consists of a hub residing on the switch card and slaves on NPs and line cards. TCU Trunk coupling unit. In Token Ring networks, a physical device that enables a station to connect to the trunk cable. TDM Time-division multiplexing. Technique in which information from multiple channels can be allocated bandwidth on a single wire based on preassigned time slots. Bandwidth is allocated to each channel regardless of whether the station has data to transmit. Compare with ATDM, FDM, and statistical multiplexing. TDR Time domain reflectometer. Device capable of sending signals through a network medium to check cable continuity, length, and other attributes. TDRs are used to find physical layer network problems. Technical Assistance Center See TAC. Technical Office Protocol See TOP. telco Abbreviation for telephone company. telecommunications Term referring to communications (usually involving computer systems) over the telephone network. Telecommunications Industry Association See TIA. telephony Science of converting sound to electrical signals and transmitting it between widely removed points. telepole Telescoping pole with a hook at one end. It is used to get cable across a ceiling or attic quickly. telex Teletypewriter service allowing subscribers to send messages over the PSTN. Telnet Command used to verify the application layer software between source and destination stations. This is the most complete test mechanism available. Tempest U.S. military standard. Electronic products adhering to the Tempest specification are designed to withstand EMP. See also EMP. TERENA Trans-European Research and Education Networking Association. Organization that promotes information and telecommunications technologies development in Europe. Formed by the merging of EARN and RARE. See also EARN and RARE. term ip netmask-format Command used to specify the format of network masks for the current session. termid SNA cluster controller identification. Termid is meaningful only for switched lines. Also called Xid. terminal Simple device at which data can be entered or retrieved from a network. Generally, terminals have a monitor and a keyboard, but no processor or local disk drive. Terminal Access Controller See TAC. Terminal Access Controller Access System See TACACS. terminal adapter Device used to connect ISDN BRI connections to existing interfaces such as EIA/TIA-232. Essentially, an ISDN modem. terminal emulation Network application in which a computer runs software that makes it appear to a remote host as a directly attached terminal. terminal server Communications processor that connects asynchronous devices such as terminals, printers, hosts, and modems to any LAN or WAN that uses TCP/IP, X.25, or LAT protocols. Terminal servers provide the internetwork intelligence that is not available in the connected devices. terminator Device that provides electrical resistance at the end of a transmission line to absorb signals on the line, thereby keeping them from bouncing back and being received again by network stations. test and control system See TCS. Texas Higher Education Network See THEnet. TFTP Trivial File Transfer Protocol. Simplified version of FTP that allows files to be transferred from one computer to another over a network. TH Transmission header. SNA header that is appended to the SNA basic information unit (BIU). The TH uses one of a number of available SNA header formats. See also FID0, FID1, FID2, FID3, and FID4. THC over X.25 Feature providing TCP/IP header compression over X.25 links, for purposes of link efficiency. THEnet Texas Higher Education Network. Regional network comprising over 60 academic and research institutions in the Texas (United States), area. Thinnet Term used to define a thinner, less expensive version of the cable specified in the IEEE 802.3 10BASE2 standard. Compare with Cheapernet. See also 10BASE2, Ethernet, and IEEE 802.3. throughput Rate of information arriving at, and possibly passing through, a particular point in a network system. TIA Telecommunications Industry Association. Organization that develops standards relating to telecommunications technologies. Together, the TIA and the EIA have formalized standards, such as EIA/TIA-232, for the electrical characteristics of data transmission. See also EIA. TIC Token Ring interface coupler. Controller through which an FEP connects to a Token Ring. tie-wraps Plastic ties used for holding cables together or for holding cables in place. time-division multiplexing See TDM. time domain reflectometer See TDR. time domain reflectometry Technique of sending an electrical signal down a cable and then timing the signal's reflection back from the end of the cable. Time Notify See TNotify. time-out Event that occurs when one network device expects to hear from another network device within a specified period of time, but does not. The resulting time-out usually results in a retransmission of information or the dissolving of the session between the two devices. Time To Live See TTL. TN3270 Terminal emulation software that allows a terminal to appear to an IBM host as a 3278 Model 2 terminal. The Cisco TN3270 implementation allows users to access an IBM host without using a special IBM server or a UNIX host acting as a server. TNotify Time Notify. Specifies how often SMT initiates neighbor notification broadcasts. See also SMT. token Frame that contains control information. Possession of the token allows a network device to transmit data onto the network. See also token passing. token bus LAN architecture using token passing access over a bus topology. This LAN architecture is the basis for the IEEE 802.4 LAN specification. See also IEEE 802.4. token passing Access method by which network devices access the physical medium in an orderly fashion based on possession of a small frame called a token. Contrast with circuit switching and contention. See also token. Token Ring Token-passing LAN developed and supported by IBM. Token Ring runs at 4 or 16 Mbps over a ring topology. Similar to IEEE 802.5. See also IEEE 802.5, ring topology, and token passing. Token Ring interface coupler See TIC. Token Ring Interface Processor See TRIP. TOP Technical Office Protocol. OSI-based architecture developed for office communications. topology Physical arrangement of network nodes and media within an enterprise networking structure. topology map Tool for managing a LightStream 2020 ATM switch that examines a network and displays the status of its nodes and trunks. The topology map is an HP OpenView-based application that runs on an NMS. TOS Type of service. Field within an IP datagram that indicates how the datagram should be handled. See COS (class of service). to switch unit See TSU. total length Field within an IP datagram that indicates total length of the header + the data. TP0 Transport Protocol Class 0. OSI connectionless transport protocol for use over reliable subnetworks. Defined by ISO 8073. TP4 Transport Protocol Class 4. OSI connection-based transport protocol. Defined by ISO 8073. trace Command that uses Time-To-Live (TTL) values to generate messages from each router used along the path. This is very powerful in its ability to locate failures in the path from the source to the destination. traffic management See ControlStream traffic management. traffic policing Process used to measure the actual traffic flow across a given connection and compare it to the total admissable traffic flow for that connection. Traffic outside of the agreed upon flow can be tagged (where the CLP bit is set to 1) and can be discarded en route if congestion develops. Traffic policing is used in ATM, Frame Relay, and other types of networks. Also know as admission control, permit processing, rate enforcement, and UPC (usage parameter control). See also tagged traffic. traffic profile Set of COS attribute values assigned to a given port on a LightStream 2020 ATM switch. The profile affects numerous parameters for data transmitted from the port including rate, cell drop eligibility, transmit priority, and inactivity timer. See also COS. traffic shaping Use of queues to limit surges that can congest a network. Data is buffered and then sent into the network in regulated amounts to ensure that the traffic will fit within the promised traffic envelope for the particular connection. Traffic shaping is used in ATM, Frame Relay, and other types of networks. Also known as metering, shaping, and smoothing. trailer Control information appended to data when encapsulating the data for network transmission. Compare with header. transaction Result-oriented unit of communication processing. transaction services layer Layer 7 in the SNA architectural model. Represents user application functions, such as spreadsheets, word-processing, or electronic mail, by which users interact with the network. Corresponds roughly with the application layer of the OSI reference model. See also data flow control layer, data link control layer, path control layer, physical control layer, presentation services layer, and transmission control layer. transceiver See MAU. transceiver cable See AUI. Trans-European Research and Education Networking Association See TERENA. transfer priority See transmit priority. transit bridging Bridging that uses encapsulation to send a frame between two similar networks over a dissimilar network. translational bridging Bridging between networks with dissimilar MAC sublayer protocols. MAC information is translated into the format of the destination network at the bridge. Contrast with encapsulation bridging. transmission control layer Layer 4 in the SNA architectural model. This layer is responsible for establishing, maintaining, and terminating SNA sessions, sequencing data messages, and controlling session level flow. Corresponds to the transport layer of the OSI model. See also data flow control layer, data link control layer, path control layer, physical control layer, presentation services layer, and transaction services layer. Transmission Control Protocol See TCP. Transmission Control Protocol/Internet Protocol See TCP/IP. transmission group In SNA routing, one or more parallel communications links treated as one communications facility. transmission header See TH. transmission link See link. transmit priority Queuing scheme in which each internal TOS of a LightStream 2020 ATM switch correlates to a relative priority in queues in the ATM network. This priority determines which traffic is serviced first in the case of contention for a network resource. Also known as forwarding priority and transfer priority. TRANSPAC Major packet data network run by France Telecom. Transparent Asynchronous Transmitter/Receiver Interface 4-byte/5-byte See TAXI 4B/5B. transparent bridging Bridging scheme often used in Ethernet and IEEE 802.3 networks in which bridges pass frames along one hop at a time based on tables associating end nodes with bridge ports. Transparent bridging is so named because the presence of bridges is transparent to network end nodes. Contrast with SRB. transport layer Layer 4 of the OSI reference model. This layer is responsible for reliable network communication between end nodes. The transport layer provides mechanisms for the establishment, maintenance, and termination of virtual circuits, transport fault detection and recovery, and information flow control. Corresponds to the transmission control layer of the SNA model. See also application layer, data link layer, network layer, physical layer, presentation layer, and session layer. Transport Protocol Class 0 See TP0. Transport Protocol Class 4 See TP4. trap Message sent by an SNMP agent to an NMS, console, or terminal to indicate the occurrence of a significant event, such as a specifically defined condition or a threshold that has been reached. See also alarm and event. tree topology LAN topology similar to a bus topology, except that tree networks can contain branches with multiple nodes. Transmissions from a station propagate the length of the medium and are received by all other stations. Compare with bus topology, ring topology, and star topology. TRIP Token Ring Interface Processor. High-speed interface processor on the Cisco 7000 series routers. The TRIP provides two or four Token Ring ports for interconnection with IEEE 802.5 and IBM Token Ring media with ports independently set to speeds of either 4 or 16 Mbps. Trivial File Transfer Protocol See TFTP. trunk Physical and logical connection between two ATM switches across which traffic in an ATM network travels. An ATM backbone is composed of a number of trunks. trunk card Line card on a LightStream 2020 ATM switch that is configured to communicate with other ATM switches. LightStream 2020 trunk cards offer a variety of interface types. CLCs, LSCs, and MSCs can operate as trunk cards. See also edge card. trunk coupling unit See TCU. trunk up-down See TUD. TSU To switch unit. Subsystem on each LightStream 2020 ATM switch line card that appends ATM routing information to outgoing cells and sends the cells to the switch card. TTL Time To Live. Field in an IP header that indicates how long a packet is considered valid. tunneling Architecture that is designed to provide the services necessary to implement any standard point-to-point encapsulation scheme. See also encapsulation. TUD Trunk up-down. Protocol used in ATM networks that monitors trunks and detects when one goes down or comes up. ATM switches send regular test messages from each trunk port to test trunk line quality. If a trunk misses a given number of these messages, TUD declares the trunk down. When a trunk comes back up, TUD recognizes that the trunk is up, declares the trunk up, and returns it to service. See also trunk. TUV German test agency that certifies products to European safety standards. twisted pair Relatively low-speed transmission medium consisting of two insulated wires arranged in a regular spiral pattern. The wires can be shielded or unshielded. Twisted pair is common in telephony applications and is increasingly common in data networks. See also STP and UTP. two-way simultaneous See TWS. TWS Two-way simultaneous. Mode that allows a router configured as a primary SDLC station to achieve better utilization of a full-duplex serial line. When TWS is enabled in a multidrop environment, the router can poll a secondary station and receive data from that station while it sends data to or receives data from a different secondary station on the same serial line. TYMNET See XStream. Type 1 operation IEEE 802.2 (LLC) connectionless operation. Type 2 operation IEEE 802.2 (LLC) connection-oriented operation. type of service See TOS.